The present invention relates to a method for manufacturing a semiconductor device such as diode, transistor, thyristor, insulated gate bipolar transistor (IGBT), MOSFET, and the like, having high speed switching characteristics as well as high electric characteristics.
In order to obtain high speed switching response characteristics with a semiconductor device having pn junction, there has been a known method for reducing the lifetime of small number of carriers by diffusing heavy metals such as gold or platinum, or by irradiating corpuscular rays like electron ray, X-ray, proton, and the like.
The method for diffusing the heavy metals such as gold or platinum on a semiconductor layer has been studied for long, but due to the difficulty of control of the diffusion amount and homogenizing, so the method involves a problem of no satisfactory correlation (hFE-tstg) between the current amplification rate (hFE) and the storage time (tstg) in the transistor or the like. Furthermore, it is necessary to take care of the prevention of soiling of apparatus such as a post-treatment furnace, and process control is also laborious.
Furthermore, the method of irradiating corpuscular ray is to reduce the lifetime of the carrier because the corpuscular ray having high energy causes a defect to the crystals in the semiconductor layer and forms a deep level. These crystal defects are bound by a restriction that they require to be treated in the course of the latter half of the semiconductor manufacturing process, as the lifetime of the carrier reinstates to the original state through the heat treatment at relatively low temperature to cause loss of effect.
An object of the present invention is to provide a method for manufacturing a semiconductor device having high speed switching characteristics in stable manner without causing problems in manufacturing process such as diffusion of heavy metal or irradiation of corpuscular ray.
As a result of the strenuous study continued by the present inventors to obtain a semiconductor device which can give high speed switching characteristics in a stabilized condition, it has been found that, by providing epitaxial growth of a semiconductor layer on a wafer not provided with mirror surface finish but having irregularity on the surface, a semiconductor layer having moderate crystal defects grows, and the crystal defects do not show much decrease even in the subsequent manufacturing process accompanied with temperature increase, and moreover, the crystal defects can be stabilized by providing heat treatments by rapid heating-up and rapid cooling-down, thereby making it possible to accelerate the switching speed without causing any problem to the electric characteristics.
The semiconductor manufacturing method of the present invention is characterized by providing epitaxial growth of a semiconductor layer on a surface of a sub-wafer not provided with mirror finishing, introducing impurities having different conductivity type in said epitaxially grown semiconductor layer to form at least a pn junction, and further providing rapid thermal anneal by rapid heating-up and rapid cooling-down in any step in a manufacturing process.
The sub-wafer not provided with mirror surface referred to here is one obtained by providing a surface treatment by wet chemical etching only after slicing to a wafer from an ingot, or, even in case of polishing with a polishing material, by making the final polishing with a polishing material larger than 2 xcexcm in the final polishing, which means the wafer to be obtained by not being provided with polishing with a polishing material of no less than 2 xcexcm in the final polishing. The term of rapid thermal anneal (RTA) by rapid heating-up and by rapid cooling-down means the heat treatment by heating with rapid temperature elevation rate and cooling with rapid temperature descending rate, which means the heat treatment capable, for example, of increasing the temperature within about 1 minute for heating from room temperature to about 850xc2x0 C. and cooling in about the same rate. The method includes a method of charging the object on a boat having small thermal capacity rapidly in a heating furnace of predetermined temperature, and a method of heating by means of optical lamp such as an infrared lamp, discharge lamp, laser beam lamp, or by high frequency irradiation.
By providing the sub-wafer surface with wet chemical treatment, or by polishing with a polishing material of more than 2 xcexcm, the surface of the above sub-wafer is formed, and by epitaxially growing the above semiconductor layer on the surface, the surface treatment of sub-wafer can be made in a short time to give a semiconductor device having rapid switching speed without requiring time-taking polishing.
Concretely, the sub-wafer surface is processed to surface roughness of 0.001-0.5 xcexcm, more preferably of 0.01-0.5 xcexcm, in a microroughness, and the heat treatment (RTA) may be carried out at 700-900xc2x0 C. The microroughness means, as shown in FIG. 3, the maximum variation amount between the peak and the trough in about 500 xcexcm length.
It is preferable for the heat treatment (RTA) by rapid heating-up and rapid cooling-down to be carried out after the annealing step in the semiconductor device manufacturing process in the sense of making the crystal defect assured. The above rapid heating-up may be carried out by irradiation of light lamp or by high frequency irradiation.
By allocating the epitaxially grown semiconductor layer to be a collector region, the region formed by introducing the impurities having different conductivity type into the semiconductor layer to be a base region, and introducing the same conductivity type impurities as the semiconductor layer into the base region to form an emitter region, a transistor having increased switching speed can be formed.